1. Field of the Invention
This invention relates generally to the measurement of corrosion rates and more particularly, the invention relates to an improved corrosion probe and method for rapidly measuring corrosion rates of metals in a corrosive or noncorrosive liquid environment where the measurements are made on-line.
2. Description of the Prior Art
Corrosion of condensate systems in boilers can cause costly component failures. Several presently used conventional condensate corrosion monitoring techniques utilize indirect methods of measurement of various processes which are indicative of the corrosivity of the environment being measured. These methods include measurements of conductivity, pH, iron or copper concentration and dissolved oxygen concentration. Because they measure indications of corrosivity only indirectly, none of these methods can provide information which may be necessary to the condensate system operator, such as the instantaneous corrosion rate.
Direct measurement of corrosion rate in a corrosive liquid environment is also an established procedure and can be performed on-line by inserting a probe into a moving or stationary liquid. One example of such a probe and method of measurement is described in U.S. Pat. No. 4,840,719, describing a process for measuring the corrosion rates of metals in a corrosive liquid environment by essentially inserting a probe into a corrosive liquid and electrically attaching the probe to a potentiostat. The measurement in a corrosive liquid, of high ionic strength, is facilitated by the rapid movement of the ions in the liquid between different electrodes in the probe.
A problem develops, however, when the corrosion rate of metals in a low ionic strength medium, such as distilled water, is being measured. Because there is a low concentration of ions in the liquid, charge cannot flow easily between the electrodes of the probe, thus rendering the measurement of the corrosion rate of the metals in contact with the liquid much more difficult and not possible within certain ranges.
For example, presently being sold in the United States is a prior art device for measuring corrosion rate of metals in liquid environments of strong or moderate ionic strength. This device includes probes and instruments which provide a direct measure of corrosion rate of metals in electrolytes by the technique of linear polarization resistance. The device interacts with the electrochemical corrosion mechanism in order to determine the rate at which metal ions are dissolving into solution. The passage of metal ions into solution is what causes corrosion of the metals.
The prior art device applies a small potential between metal electrodes and measures current flowing between the electrodes. The current depends on both the rate of the corrosion reaction and the resistance of the medium. Without a certain amount of ionic strength providing for conductivity in the medium, the instruments of the prior art devices cannot operate. Sale of the prior art devices, in fact, is often accompanied by sales literature which includes a graph indicating a nonoperative region of the device where the ionic conductivity of the medium is low, such as for rain water or distilled water or even boiler condensate water, as the medium in which corrosion is measured.
The need and desirability is recognized of a corrosion measuring instrument that is not limited by the lack of conductivity of the medium in which the measurements are being made.